Channel sounding techniques for a wireless communication system

ABSTRACT

A technique for channel sounding in a wireless communication system includes determining respective geometries of multiple subscriber stations with respect to a serving base station. Respective time periods for sounding a channel between the multiple subscriber stations and the serving base station are then set based on the respective geometries of the multiple subscriber stations.

BACKGROUND

1. Field

This disclosure relates generally to channel sounding, and morespecifically, to channel sounding techniques for a wirelesscommunication system.

2. Related Art

In general, coded orthogonal frequency division multiplexing (COFDM)systems support high data rate wireless transmission using orthogonalchannels, which typically offer immunity against fading and inter-symbolinterference (ISI) without requiring implementation of elaborateequalization techniques. Typically, COFDM systems split data into Nstreams, which are independently modulated on parallel spaced subcarrierfrequencies or tones. The frequency separation between subcarriers is1/T, where T is the COFDM symbol time duration. Each symbol may includea guard interval (or cyclic prefix) to maintain the orthogonality of thesymbols. In general, COFDM systems have utilized an inverse discreteFourier transform (IDFT) to generate a sampled (or discrete) compositetime-domain signal. One undesirable attribute of COFDM systems is thatthey may exhibit relatively large peak-to-average power ratio (PAR),when signals from different subcarriers add constructively. A large PAR(and/or large cubic metric (CM)) is undesirable as it requires a largedynamic range for a digital-to-analog converter (DAC) implemented withina transmitter of a COFDM system. Consequently, the DAC may be usedinefficiently as most subcarrier amplitudes use a fraction of the rangeof the DAC.

In a typical implementation, the output of the DAC is filtered beforebeing applied to a power amplifier. As power amplifiers tend to benon-linear, in-band distortion and spectral spreading (or spectralregrowth) may occur. As is known, spectral regrowth may occur when aband-limited time-varying (non-constant) envelope signal is passedthrough a non-linear circuit. One technique for addressing non-linearityof a power amplifier has operated the power amplifier at a relativelylarge output power back-off (OBO). Unfortunately, operating a poweramplifier at a relatively large OBO (or power de-rating) reduces thepower efficiency of the amplifier. For example, at a 6 dB (decibel) OBO,a power amplifier may exhibit a fifty percent (or more) loss inefficiency. To reduce the PAR and/or CM of COFDM systems, variousdesigners have also implemented or proposed hard limiting (or clipping)directly on the signal to be transmitted. Unfortunately, directlyclipping the signal to be transmitted may cause undesirable spectralregrowth and inter-user interference (or inter-carrier interference(ICI)) in systems that utilize multiple access mode.

Discrete Fourier transform-spread orthogonal frequency divisionmultiplexing (DFT-SOFDM) has been proposed as the modulation techniquefor the uplink of evolved-universal terrestrial radio access (E-UTRA).Single carrier transmission schemes, such as DFT-SOFDM, generallyfacilitate further power de-rating reduction through the use of, forexample, specific modulation or coding schemes, or clipping and spectralfiltering of a signal to be transmitted. Moreover, the PAR and CM of abasic DFT-SOFDM (or single carrier-frequency division multiple access(SC-FDMA)) system is generally reduced, as compared to the PAR and CM ofa basic COFDM system. To further reduce the PAR and CM of basicDFT-SOFDM transmitters, one group of designers has proposedpre-processing an input signal prior to performing a fast Fouriertransform (FFT) on a group of symbols associated with the input signal.Following this approach, selected input symbols and/or bits may beattenuated in order to reduce the PAR and CM at the output of an inversefast Fourier transform (IFFT) of the DFT-SOFDM system.

In general, wireless networks have used an estimated received signalstrength and an estimated carrier to interference and noise ratio (CINR)of a received signal to determine operational characteristics of thenetworks. As one example, IEEE 802.16e compliant mobile stations (MSs)are required to estimate a received signal strength indicator (RSSI) anda CINR of a received signal. In general, CINR at an MS may be calculatedas the ratio of an RSSI of a serving base station (BS) to summed RSSIsof non-serving BSs added to a white noise power of a receiver of the MS.The RSSI associated with a serving BS may be used by an MS for uplinkpower control and the CINR, which is reported to the serving BS, may beused by the serving BS to adapt a downlink transmission rate to linkconditions.

Accurate reported CINRs are desirable, as inaccurate reported CINRs mayimpact performance of a wireless network. For example, reporting a CINRthat is above an actual CINR may decrease network throughput due toframe re-transmission, while reporting a CINR that is below the actualCINR may cause the serving BS to schedule data rates below a supportabledata rate. According to IEEE 802.16e, RSSI and CINR estimates at an MSare derived based on a preamble signal, which is an orthogonal frequencydivision multiple access (0FDMA) symbol that is transmitted at thebeginning of each OFDMA frame.

Similarly, wireless networks that employ third-generation partnershipproject long-term evolution (3GPP-LTE) compliant architectures arerequired to employ uplink reference signals (RSs), which are scheduledto user equipment (subscriber stations (SSs)) within a 3GPP-LTE network.Respective sequences of the RSs are used to uniquely identify an SS and,when transmitted from the SS to a serving base station (BS), may be usedby the serving BS in channel characterization. Known channel sounding(channel characterization) approaches have sounded a channel over arelatively long time period, irrespective of geometries of subscriberstations (SSs) with respect to a serving base station (BS). In wirelessnetworks employing known channel sounding approaches, channel soundingbandwidth may consume a relatively large portion of an uplink (UL)channel.

What is needed are techniques for reducing channel sounding bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 is a diagram of an example uplink (UL) subframe that includes ademodulation reference signal (RS) positioned in a fourth (middle)symbol of each of two slots.

FIG. 2 is a flowchart of a channel sounding assignment process that maybe, at least partially, employed in a scheduler in a wirelesscommunication system, according to the present disclosure.

FIG. 3 is a flowchart of a process for receiving and transmitting anassigned channel sounding burst from a given subscriber station (SS) ina wireless communication system, according to the present disclosure.

FIG. 4 is a flowchart of a carrier to interference and noise ratio(CINR) determination process that may be employed in base stations (BSs)of a wireless communication system, according to another embodiment ofthe present disclosure.

FIG. 5 is a block diagram of an example wireless communication systemthat may assign channel sounding RSs to SSs according to variousembodiments of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of theinvention, specific exemplary embodiments in which the invention may bepracticed are described in sufficient detail to enable those skilled inthe art to practice the invention, and it is to be understood that otherembodiments may be utilized and that logical, architectural,programmatic, mechanical, electrical and other changes may be madewithout departing from the spirit or scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims. In particular, although the preferred embodimentis described below with respect to a wireless mobile communicationdevice, it will be appreciated that the present invention is not solimited and that it has application to other embodiments of wirelesselectronic devices such as personal digital assistants (PDAs), digitalcameras, portable storage devices, audio players, computer systems, andportable gaming devices, for example.

As is used herein, the term “user equipment” is synonymous with the term“subscriber station,” which is used to denote a wireless device.According to one aspect of the present disclosure, an uplink (UL)channel sounding scheme is employed that generally reduces UL channelsounding resources, e.g., channel sounding bandwidth. The UL channelsounding scheme may enable low-geometry SSs (i.e., SSs with relativelylow CINRs) to sound a channel over a longer period of time (e.g., overchannel sounding symbols provided in multiple long blocks (LBs)) andhigh geometry SSs (i.e., SSs with relatively high CINRs) to sound thechannel in one LB. As noted above, conventional channel sounding schemeshave set a fixed relatively long time period over which all SSs, incommunication with a given serving base station (BS), have sounded achannel, i.e., a number of subcarriers over which an SS communicateswith the serving BS. In general, when all SSs sound a channel over afixed relatively long time period (averaging multiple channel soundingsymbols), channel sounding bandwidth requirements are relatively large.A geometry of an SS with respect to a serving BS may be determined by,for example, determining a CINR associated with the SS. For example,power-limited subscriber stations (SSs), e.g., SSs that are near acell-edge, may have a relatively low associated CINR at a serving BS,and thus, be classified as a low geometry SS.

With reference to FIG. 1, an example uplink (UL) subframe includes areference signal (RS) positioned in a fourth (middle) symbol of eachslot. In the illustrated example, a UL subframe includes two slots, eachof which include seven LBs and which encode a symbol. It should beappreciated that the techniques disclosed herein are broadly applicableto UL subframes that employ more or less than the illustrated number ofLBs. In the example UL subframe depicted in FIG. 1, the RS in each ofslots 0 and 1 are demodulation RSs. According to various embodiments ofthe present disclosure, channel sounding RSs may be scheduled in any ofthe LBs in either slot of the UL subframe. According to one or moreembodiments, channel sounding symbols scheduled in a same LB of asubframe are configured to be orthogonal when the channel soundingsymbols are assigned to same channel (or subchannel). That is, whenmultiple SSs are scheduled to transmit channel sounding symbols over thesame channel (i.e., group of subcarriers), the scheduled channelsounding symbols for each of the multiple SSs are configured as codedivision multiplexed (CDM) sequences. The CDM sequences may be generatedby cyclic shift of one or more base sequences. In general, a length ofthe cyclic shift may be based on a typical time delay spread associatedwith the SSs in a cell. For example, in a wireless communication systemhaving a typical time delay spread of five microseconds and a samplingfrequency of 7.68 MHZ, a cyclic shift of forty may be employed. The CDMsequences may be, for example, constant amplitude zero autocorrelation(CAZAC) sequences, generated in a number of ways. The generation of theCOM sequences is not particularly relevant to the present disclosureand, as such, is not discussed further herein.

As previously noted, to differentiate SSs (and/or cells), multipleunique RSs are implemented within a wireless communication system. Asnoted, different RSs usually have different CMs. RSs with relativelyhigh CMs may require output power backoff (OBO) of a power amplifier ofan associated SS transmitter to avoid non-linear distortion. As isknown, employing OBO in a power amplifier of a transmitter lowers thepower efficiency of the power amplifier. Power-limited SSs may beindicated by SSs that are operating at or near maximum transmitter power(e.g., SSs with a transmitter power of 24 dBm (decibels with respect toone milliwatt). According to the present disclosure, bandwidth forchannel sounding may usually be reduced by determining respectivegeometries of multiple SSs with respect to a serving BS. Respective timeperiods for sounding a channel with a channel sounding symbol (orsymbols) is then set based upon the respective geometries of individualSSs with respect to a serving BS. For example, a channel for ahigh-geometry SS (e.g., an SS with a CINR of about 15 dB) may becharacterized based upon the transmission of one channel sounding symbolfrom the high-geometry SS. As another example, a channel for alow-geometry SS (e.g., an SS with a CINR of about 0 dB) may becharacterized based upon the transmission of five channel soundingsymbols from the low-geometry SS. As yet another example, a channel fora medium-geometry SS (e.g., an SS with a CINR of about 7.5 dB) may becharacterized based upon the transmission of three channel soundingsymbols from the medium-geometry SS. In the event that multiple soundingsymbols are transmitted from an SS, the serving BS averages themeasurements across the multiple channel sounding symbols. It should beappreciated that the CINRs, set forth above, are example CINRs.

In general, a length of an RS (r_(u)(n)) is determined by a length of adiscrete Fourier transform (DFT), e.g. a fast Fourier transform (FFT),that is used for the RS (i.e., the number of subcarriers employed). Forexample, when an RS is assigned one resource block (i.e., twelvesubcarriers in the frequency-domain), ten basis sequences may begenerated using a cyclic extension approach, i.e., r_(u)(n), 1≦u≦10,0≦n≦NFFT−1, where NFFT is the size of the DFT. From each basis, twelveorthogonal sequences may be generated using a cyclic shift in thefrequency-domain. An uplink transmitter may implement one of a phaseshift keying (PSK), a quadrature amplitude modulation (QAM), or otherdata modulation scheme, depending upon which modulation scheme isscheduled. It should be appreciated that any of the various PSK, e.g.,pi/2 BPSK, QPSK and 8-PSK, or QAM, e.g., 16-QAM and 64-QAM, modulationtechniques may be implemented in a wireless communication systemconstructed according to the present disclosure.

In the disclosed approach, the serving BS initially calculates a CINR ofa training signal transmitted from a given SS, during a trainingsequence, and compares the calculated CINR to one or more thresholds todetermine a geometry of the given SS. The training signal may be, forexample, a random access preamble or a channel sounding burst. In theevent that the given SS is determined to be a low-geometry SS, a channelsounding RS may be assigned, by a scheduler, e.g., a network scheduler,to the SS for transmission multiple times over a relatively long timeperiod. In the event that the SS is later detected to be at a highergeometry, the scheduler may reduce the number of times that the SS isscheduled to transmit the channel sounding RS, in another channelsounding period. The time period over which the SS is scheduled totransmit the channel sounding RS should generally be less than acoherence time of the UL channel a time over which the UL channel isstable). Moreover, a bandwidth assigned to the channel sounding RSincludes enough subcarriers such that code division multiplexing (CDM)can be employed for the channel sounding RSs transmitted by the SSs (forexample, twelve subcarriers are typically required to implement CDM forthe UL channel). The channel sounding symbols transmitted by thedifferent SSs should usually be orthogonal, such that multiple SSs cantransmit channel sounding RSs simultaneously over the same channel(group of subcarriers) without interference. A serving BS can thenreceive the respective channel sounding RSs transmitted by respectiveSSs and accurately determine channel characteristics based on thereceived channel sounding RSs. For example, for SSs that are relativelyclose to a serving BS, one channel sounding symbol may generally sufficeto provide relatively accurate CINR calculations.

For power-limited SSs (e.g., cell-edge SSs that are transmitting at apower level of about 24 dBm) that are farther from the serving BS, achannel sounding symbol may be scheduled to be transmitted by an SSmultiple times within an averaging time period. The serving BS thenintegrates the received channel sounding symbols to provide relativelyaccurate CINR calculations. To improve noise and interference estimates,one or more blank cyclic shifts may be employed. That is, certain of theCDM sequences (blank cyclic shifts) may not be assigned to an SS. Inthis manner, a serving BS may estimate noise and interference based onthe decoded blank cyclic shift(s).

Turning to FIG. 2, a process 200 for determining a channel sounding timeperiod for SSs is depicted. The process 200 may be predominantlyemployed in a scheduler, e.g., a network-based scheduler, of a wirelesscommunication system. The process 200 is initiated at block 202, atwhich point control transfers to decision block 204. In block 204, theserving base station (BS) determines whether a training signal has beenreceived from an SS. If a training signal is received in block 204,control transfers to block 206, where a CINR for the SS is determinedbased on the training signal. If a training signal is not received inblock 204, control loops on block 204 until a training signal isreceived. Following block 206, control transfers to decision block 208where it is determined, e.g. by a scheduler, whether the CINR is lessthan a threshold(s). If the CINR of an SS is less than a threshold,control transfers to block 212, where a channel sounding time period forthe SS is set for multiple channel sounding symbols. If the CINR of anSS is not less than a threshold, control transfers to block 210, where achannel sounding time period is set for a single channel soundingsymbol. In alternative embodiments, multiple thresholds may be employedwhen determining the number of channel sounding symbols. Followingblocks 210 and 212, control transfers to block 214. In block 214, theserving BS transmits the channel sounding schedule to the SS. Next, indecision block 216, it is determined whether additional SSs requiretraining. If so control transfers to block 204. Otherwise, controltransfers from block 216 to block 218, where control returns to acalling routine.

A CINR of the received signal may be estimated through a number ofapproaches. As a first example, U.S. Patent Application Publication No.2006/0133260 discloses a channel estimation based approach forestimating CINR that isolates noise and interference components usingpilot sequences and estimates a channel power by subtracting a combinednoise and interference power estimate from a received power estimate. Asa second example, U.S. Patent Application Publication No. 2006/0093074discloses a difference based approach for estimating CINR that assumesthat adjacent pilot locations have the same subchannel characteristics.In view of this assumption, noise and interference components areisolated by subtracting adjacent received signals.

Moving to FIG. 3, a process 300 for receiving an assigned channelsounding burst schedule and transmitting an assigned channel soundingburst is illustrated. In block 302, the process 300 is initiated, atwhich point control transfers to block 304. In block 304, an assignedchannel sounding burst schedule is received by an SS. Next, in block306, the assigned channel sounding burst (including one or more channelsounding symbols) is transmitted by the SS. Following block 306, controltransfers to block 308, where control returns to a calling routine. Ingeneral, providing multiple channel sounding symbols from a low-geometrySS (e.g., a cell-edge SS) allows a serving BS to increase the accuracyof an estimated CINR for the low-geometry SS and, thus, generallyimproves performance of an associated wireless communication system.

Turning to FIG. 4, a CINR determination process 400, that may beemployed in a serving BS of a wireless communication system, isdepicted. The process 400 may be utilized to determine a CINR of atraining signal (or a non-training signal) and is initiated at block402, at which point control transfers to block 404. In block 404, theserving BS receives transmitted channel sounding symbols from multipleSSs. Next, in block 406, the received channel sounding symbols aredecoded. Then, in block 408, CINRs are determined for the SSs using thedecoded channel sounding symbols and, for example, one or more blankcyclic shifts. As noted above, a blank cyclic shift is a sequence thatwas not assigned to an SS for transmission. The blank cyclic shiftallows the serving BS to make a determination of the interference andnoise (I+N) level on a channel. In general, the interference may beattributed to SSs in adjacent cells and the noise is white noiseattributable to a receiver of the serving BS. Next, in block 410,control returns to a calling routine.

With reference to FIG. 5, an example wireless communication system 500is depicted that includes a plurality of wireless devices (subscriberstations) 502, e.g., hand-held computers, personal digital assistants(PDAs), cellular telephones, etc., that may implement communicationlinks according to one or more embodiments of the present disclosure. Ingeneral, the wireless devices 502 include a processor 508 (e.g., adigital signal processor (DSP)), a transceiver 506, and one or moreinput/output devices 504 (e.g., a camera, a keypad, display, etc.),among other components not shown in FIG. 5. As is noted above, accordingto various embodiments of the present disclosure, a technique isdisclosed that facilitates channel sounding for a wireless communicationdevice, such as the wireless devices 502. The wireless devices 502communicate with a base station controller (BSC) 512 of a base stationsubsystem (BSS) 510, via one or more base transceiver stations (BTS)514, to receive or transmit voice and/or data. The BSC 512 may, forexample, be configured to schedule communications for the wirelessdevices 502.

The BSC 512 is also in communication with a packet control unit (PCU)516, which is in communication with a serving general packet radioservice (GPRS) support node (SGSN) 522. The SGSN 522 is in communicationwith a gateway GPRS support node (GGSN) 524, both of which are includedwithin a GPRS core network 520. The GGSN 524 provides access tocomputer(s) 526 coupled to Internet/intranet 528. In this manner, thewireless devices 502 may receive data from and/or transmit data tocomputers coupled to the Internet/intranet 528. For example, when thedevices 502 include a camera, images may be transferred to a computer526 coupled to the Internet/intranet 528 or to another one of thedevices 502. The BSC 512 is also in communication with a mobileswitching center/visitor location register (MSC/VLR) 534, which is incommunication with a home location register (HLR), an authenticationcenter (AUC), and an equipment identity register (MR) 532. In a typicalimplementation, the MSC/VLR 534 and the HLR, AUC, and EIR 532 arelocated within a network and switching subsystem (NSS) 530, which mayalso perform scheduling for the system 500. The SGSN 522 may communicatedirectly with the HLR, AUC and EIR 532. As is also shown, the MSC/VLR534 is in communication with a public switched telephone network (PSTN)542, which facilitates communication between wireless devices 502 andland telephones 540.

Accordingly, a number of techniques have been disclosed herein thatgenerally reduce channel sounding bandwidth in a wireless communicationsystem. While the discussion herein has been primarily directed toimproving communications on an uplink of a wireless communicationsystem, it is contemplated that many of the techniques disclosed hereinare equally applicable to improving communications on a downlink of awireless communication system.

As used herein, a software system can include one or more objects,agents, threads, subroutines, separate software applications, two ormore lines of code or other suitable software structures operating inone or more separate software applications, on one or more differentprocessors, or other suitable software architectures.

As will be appreciated, the processes in preferred embodiments of thepresent invention may be implemented using any combination of computerprogramming software, firmware or hardware. As a preparatory step topracticing the invention in software, the computer programming code(whether software or firmware) according to a preferred embodiment willtypically be stored in one or more machine readable storage mediums suchas fixed (hard) drives, diskettes, optical disks, magnetic tape,semiconductor memories such as read-only memories (ROMs), programmableROMs (PROMs), etc., thereby making an article of manufacture inaccordance with the invention. The article of manufacture containing thecomputer programming code is used by either executing the code directlyfrom the storage device, by copying the code from the storage deviceinto another storage device such as a hard disk, random access memory(RAM), etc., or by transmitting the code for remote execution. Themethod form of the invention may be practiced by combining one or moremachine-readable storage devices containing the code according to thepresent invention with appropriate standard computer hardware to executethe code contained therein. An apparatus for practicing the inventioncould be one or more computers and storage systems containing or havingnetwork access to computer program(s) coded in accordance with theinvention.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. For example, the channel sounding techniques disclosedherein are generally broadly applicable to wireless communicationsystems. Accordingly, the specification and figures are to be regardedin an illustrative rather than a restrictive sense, and all suchmodifications are intended to be included with the scope of the presentinvention. Any benefits, advantages, or solution to problems that aredescribed herein with regard to specific embodiments are not intended tobe construed as a critical, required, or essential feature or element ofany or all the claims.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

1-20. (canceled)
 21. An apparatus for use in a wireless communicationsystem, comprising: a processor; and a computer readable apparatushaving a storage medium with at least one computer program storedthereon, the at least one computer program comprises instructions, whichwhen executed on the processor: determine respective geometries ofmultiple subscriber stations with respect to a serving base station;set, based on said respective geometries, respective time durations forsounding a channel between each of the multiple subscriber stations andthe serving base station.
 22. The apparatus of claim 21, wherein thecomputer program further comprises instructions, which when executed bythe processor: set, based on the determination, a first time durationfor sounding the channel between a first subscriber station, includedwithin the multiple subscriber stations, and the serving base station;and set, based on the determination, a second time duration for soundingthe channel between a second subscriber station, included within themultiple subscriber stations, and the serving base station, wherein thesecond time duration is greater than the first time duration and thesecond subscriber station is located at a lower geometry with respect tothe serving base station than the first subscriber station.
 23. Theapparatus of claim 21, wherein the computer program further comprisesinstructions, which when executed by the processor: assign respectivechannel sounding burst schedules to the multiple subscriber stations forthe channel over respective integration time periods that are less thana coherence time of the channel, wherein a number of subcarriersassigned to the channel is not less than a minimum number forimplementing code division multiplexing for the multiple subscriberstations assigned to the channel; and communicate, to the multiplesubscriber stations, the respective channel sounding burst schedules.24. The apparatus of claim 21, wherein the computer program furthercomprises instructions, which when executed by the processor: receive,from the multiple subscriber stations, respective channel soundingbursts on the channel based on the respective channel sounding burstschedules.
 25. The apparatus of claim 24, wherein the computer programfurther comprises instructions, which when executed by the processor:determine a carrier to interference and noise ratio associated with eachof the multiple subscriber stations based on one or more blank cyclicshifts and respective orthogonal channel sounding symbols of therespective received channel sounding bursts.
 26. The apparatus of claim24, wherein the computer program further comprises instructions, whichwhen executed by the processor: characterize, for the multiplesubscriber stations, the channel based on received respective channelsounding bursts from the multiple subscriber stations, wherein thecharacterization includes a determined carrier to interference and noiseratio for each of the multiple subscriber stations.
 27. The apparatus ofclaim 21, wherein the respective time durations correspond to onechannel sounding symbol for at least a first one of the multiplesubscriber stations and to multiple channel sounding symbols for atleast a second one of the multiple subscriber stations.